Slide gates are parts used in continuous casting of steel for opening and closing distributors or outlet orifices from casting ladles in fluid communication via a sliding nozzle with ingot molds. Slide gates must therefore present good mechanical strength, in particular against thermal shocks and spalling, and good chemical resistance, in particular against corrosion.
Conventionally, slide gates are obtained by sintering a mixture of alumina-zirconia fused grains and zirconia-mullite fused grains. The alumina-zirconia/zirconia-mullite composite is particularly good at withstanding thermal shocks because of its reinforcement by microcracking. During heating, the allotropic transformation of zirconia is accompanied by a large change in volume. This dimensional variation leads to the formation of microcracks. These microcracks also appear at the interfaces between zirconia-mullite particles and the alumina-zirconia matrix because of the large difference in thermal expansion between alumina-zirconia (α1000°C.=9.6×10−6/° C.) and zirconia-mullite (α1000°C.=6.9×10−6/° C.). These two phenomena lead to the part being microcracked, thereby increasing its capacity for absorbing energy in the event of thermal shocks.
Nevertheless, zirconia-mullite presents the disadvantage of presenting low resistance to corrosion, thus constituting the weak point of the composite.